The present disclosure relates to a switchable roller finger follower for a valve train of an internal combustion (IC) engine, and more particularly, to the coupling pin of a switchable roller finger follower (SRFF) that provides at least two discrete valve lift modes.
More stringent fuel economy regulations in the transportation industry have prompted the need for improved efficiency of the IC engine. Light-weighting, friction reduction, thermal management, variable valve timing and a diverse array of variable valve lift technologies are all part of the technology toolbox for IC engine designers.
Variable valve lift (VVL) systems typically employ a technology in a valve train of an IC engine that allows different engine valve lifts to occur. The valve train is formed of the components that are required to actuate an engine valve, including a camshaft (also termed “cam”), the valve, and all components that lie in between. VVL systems are typically divided into two categories: continuous variable and discrete variable. Continuous variable valve lift systems are capable of varying a valve lift from a design lift minimum to a design lift maximum to achieve any of several lift heights. Discrete variable valve lift systems are capable of switching between two or more distinct valve lifts. Components that enable these different valve lift modes are often called switchable valve train components. Typical two-step discrete valve lift systems switch between a full valve lift mode and a partial valve lift mode, often termed cam profile switching, or between a full valve lift mode and a no valve lift mode that facilitates deactivation of the valve. Three-step discrete valve lift systems can combine valve deactivation and cam profile switching strategies. Valve deactivation can be applied in different ways. In the case of a four-valve-per-cylinder configuration (two intake+two exhaust), one of two intake valves can be deactivated. Deactivating only one of the two intake valves can provide for an increased swirl condition that enhances combustion of the air-fuel mixture. In another scenario, all of the intake and exhaust valves are deactivated for a selected cylinder which facilitates cylinder deactivation. On most engines, cylinder deactivation is applied to a fixed set of cylinders, when lightly loaded at steady-state speeds, to achieve the fuel economy of a smaller displacement engine. A lightly loaded engine running with a reduced amount of active cylinders requires a higher intake manifold pressure, and, thus, a greater throttle plate opening, than an engine running with all of its cylinders in the active state. Given the lower intake restriction, throttling losses are reduced in the cylinder deactivation mode and the engine runs with greater efficiency. For those engines that deactivate half of the cylinders, it is typical in the engine industry to deactivate every other cylinder in the firing order to ensure smoothness of engine operation while in this mode. Deactivation also includes shutting off the fuel to the dormant cylinders. Reactivation of dormant cylinders occurs when the driver demands more power for acceleration. The smooth transition between normal and partial engine operation is achieved by controlling ignition timing, cam timing and throttle position, as managed by the engine control unit (ECU). Examples of switchable valve train components that serve as cylinder deactivation facilitators include roller finger followers, roller lifters, pivot elements, rocker arms and camshafts; each of these components is able to switch from a full valve lift mode to a no valve lift mode. The switching of lifts occurs on the base circle or non-lift portion of the camshaft; therefore the time to switch from one mode to another is limited by the time that the camshaft is rotating through its base circle portion; more time for switching is available at lower engine speeds and less time is available at higher engine speeds. Maximum switching engine speeds are defined by whether there is enough time available on the base circle portion to fully actuate a coupling assembly to achieve the desired lift mode.
In today's IC engines, many of the switchable valve train components that enable valve deactivation for cylinder deactivation contain a coupling or locking assembly that is actuated by an electro-hydraulic system. The electro-hydraulic system typically contains at least one solenoid valve within an array of oil galleries that manages engine oil pressure to either lock or unlock the coupling assembly within the switchable valve train component to enable a valve lift switching event. These types of electro-hydraulic systems require time within the combustion cycle to actuate the switchable valve train component.
In most IC engine applications, switchable valve train components for cylinder deactivation in an electro-hydraulic system are classified as “pressure-less-locked”, which equates to:
a). In a no or low oil pressure condition, the spring-biased coupling assembly will be in a locked position, facilitating the function of a standard valve train component that translates rotary camshaft motion to linear valve motion; and,
b). In a condition in which engine oil pressure is delivered to the coupling assembly that exceeds the force of the coupling assembly bias spring, the coupling assembly will be displaced by a given stroke to an unlocked position, facilitating valve deactivation where the rotary camshaft motion is not translated to the valve.
“Pressure-less-unlocked” electro-hydraulic systems can be found in some cam profile switching systems that switch between a full or high valve lift and a partial or low valve lift, which equates to:
a). In a no or low oil pressure condition, the spring-biased coupling assembly will be in an unlocked position, facilitating a partial valve lift event; and,
b). In a condition in which engine oil pressure is delivered to the coupling assembly that exceeds the force of the coupling assembly bias spring, the coupling assembly will be displaced a given stroke to a locked position, facilitating a full valve lift event.
Vital to the durability and performance of a switchable valve train component is the robustness of the coupling assembly. Two important design attributes of the coupling assembly include: 1). the ability to switch from a locked to an unlocked position very quickly, and 2). a high resistance to wear. However, many times these attributes are in opposition. For example, the locking/unlocking stroke of the coupling assembly to engage/disengage an adjacent component has a direct impact on switching times; a shorter stroke for a given cross-sectional area of a coupling assembly will likely yield a faster switching time. Yet, a shorter stroke typically dictates a smaller contact area with the engaged or disengaged component, meaning that a given load is applied over a smaller area leading to higher contact pressures and subsequent wear. For this reason, various coupling assembly forms, materials, coatings and heat treatments are often employed in an effort to maximize wear resistance in order to minimize the actuation stroke and resultant contact area.
Many coupling assembly designs utilize a coupling pin that is configured with a locking surface that engages or disengages another locking surface to enable different valve lift modes. In the case of the SRFF, the coupling pin moves longitudinally within a bore of one lever to engage or disengage another lever. In many instances the coupling pin contains a flat locking surface that engages a corresponding flat locking surface. Flat locking surfaces are used because of their increased contact area and thus lower stresses and resultant wear, as compared to other shaped interfaces. However, alignment of the flat locking surface of the locking pin with the corresponding flat locking surface is required to enable locking functionality. Therefore, a solution is needed to provide alignment or anti-rotation of the locking pin, such that its flat locking surface maintains alignment with a corresponding flat locking surface. Additionally, a solution is needed that can be applied to different known SRFF designs that facilitate valve deactivation, cam profile switching, or a combination of the two, with a compact arrangement.